Thin film battery

ABSTRACT

The present invention concerns a flat battery comprising a package formed by a cathode, an anode, and a separator layer sandwiched between the cathode and the anode, a sealing frame extending circumferentially around said package, a first current collector contacting the anode, and a second current collector contacting the cathode. The first and second current collectors each partly cover the sealing frame in a zone being adjacent to the package. According to the invention, the battery further comprises a first polymeric jacket layer being arranged on the first current collector and a second polymeric jacket layer being arranged on the second current collector, said first and second polymeric jacket layers extending circumferentially beyond the current collectors and beyond the sealing frame and being sealed together to form an outer jacket for the battery. Furthermore, the present invention also concerns a method to produce such a battery.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 13/061,844, filed Apr. 18, 2011, which is a National Stage ofInternational Application No. PCT/EP2008/061816 filed on Sep. 5, 2008,the contents of all of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

The present invention relates in general to thin film primary batteries.

Electrochemical elements, i.e. batteries, are known in many differentphysical forms. In most cases, they have a mechanically robust housingand are in the form of round, button or prismatic cells. A positive anda negative electrode, a separator and an electrolyte are arranged insuch a cell. The housing of this type of cells is in general composed ofsteel, a stainless steel alloy or aluminum.

However, for certain applications very thin batteries having a flexiblehousing are needed. These applications include active radio frequencyidentification (RFID) tags, PCMCIA cards, smart cards, etc. A batterywhich shall be used in such applications must be flexible and compact,deliver high energy density and specific energy with a low rate of selfdischarge, and should be provided with a reliable sealing. The sealingis extremely important, as moisture should be prevented from enteringinto the battery to avoid drain and self discharge, as well as toprevent drying out resulting from loosing the organic solvents in theelectrolyte, even when the battery is distorted or under mechanicalstress. Furthermore, the battery should be manufacturable in acost-efficient and reliable way.

Different thin film batteries are known from prior art, most of themusing lithium as the anode material.

U.S. Pat. No. 5,989,751 for example, discloses a primary lithium batteryhaving a flexible and compact design. The cell is provided with anelectrolyte-containing composite cathode. A packaging using a spacer andpolymeric sheets is provided.

U.S. 2003/0228517 A1 shows a thin cell with a packaging being formed bytwo plastic sheets sealed to each other. The thin cells described inthis document are stacked to form a larger electrochemical element. Theplastic sheets are metallized in certain areas to form the electricalcontacts with the electrodes of the cell.

U.S. 2005/0239917 also discloses a thin film lithium battery, whereinthe anode is printed on a copper current collector using lithium metalpowder based ink. The anode and cathode current collectors are sealedaround the perimeter of the battery by a polyester sealant frame.

U.S. Pat. No. 6,752,842 B2 discloses a thin film cell which ismanufactured by printing different layers on top of each other.

In terms of quality of the sealing and cost-efficiency upon production,the above described batteries are not yet completely satisfying.

SUMMARY OF THE INVENTION

The present invention relates to a flat battery comprising a packageformed by a cathode, an anode, and a separator layer sandwiched betweenthe cathode and the anode. A sealing frame extends circumferentiallyaround said package. The battery further comprises a first currentcollector contacting the anode and a second current collector made of ametal foil and contacting the cathode, wherein the first and secondcurrent collectors each partly cover the sealing frame in an area beingadjacent to the package.

Said battery further comprises a first polymeric jacket layer beingarranged on the first current collector and a second polymeric sheetbeing arranged on the second current collector, said first and secondpolymeric sheets extending circumferentially beyond the currentcollectors and beyond the sealing frame and being sealed together toform an outer jacket for the battery.

The present invention also relates to a method for making a flat batterycomprising the following steps:

-   -   (a) providing a first polymeric jacket layer,    -   (b) providing a first current collector and arranging same on        said first polymeric sheet,    -   (c) applying an anode material on said first current collector,    -   (d) providing a second polymeric jacket layer,    -   (e) providing a second current collector and arranging same on        said second polymeric jacket layer,    -   (f) providing a sealing frame having an inner contour        corresponding substantially to an outer contour of the anode        material and the separator on the first current collector,    -   (g) arranging said frame on one of the current collectors, the        frame covering the outer circumference of the current collector,    -   (h) providing a cathode material and applying same on the second        current collector,    -   (i) providing a separator layer and arranging same on the        cathode material,    -   (j) assembling the flat battery by returning one of the two        polymeric jacket layer and arranging it and on the other        polymeric jacket layer such that the separator layer is        sandwiched between the cathode material and the anode material,        the sealing frame then extending circumferentially around the        anode material, the cathode material and the separator, and    -   (k) sealing together said first and second polymeric jacket        layer in a zone extending circumferentially beyond the current        collectors to form an outer jacket for the battery package.

The first and second polymeric jacket layers which are sealed togetherto form an outer jacket for the battery help to avoid that water orother liquids enter the cell and establish a conductive path which mayresult in high self-discharge rates. Furthermore, electrolyte isprevented from escaping the battery, and a dry-out of the battery isthereby avoided. The sealing frame extending circumferentially aroundthe package formed by the anode, the cathode, the separator and theelectrolyte, further contributes to the high quality of the sealing. Dueto the fact that the sealing frame extends circumferentially around saidpackage, while the first and second polymeric sheets extendcircumferentially beyond the frame, a double sealing is provided. Thecurrent collectors partly covering the sealing frame form a firstsealing zone and the polymeric sheets extending circumferentially beyondsaid sealing area form a second additional sealing zone.

According to a preferred embodiment of the invention, the sealing frameis coated with a heat-sealable material. The sealing frame itself can bemade of a polymer such as nylon, polyester (PET), polypropylene or anysuitable polymer, in particular of PET (poly(ethylene terephthalate), aresin in the polyester family). The coating may consist of hot-meltadhesive EVA (ethylene vinyl acetate) or EMA (ethyl methyl acrylate), oranother suitable heat-sealable material. Due to the coating, the framecan easily be sealed to the current collectors and the polymeric sheets.The sealing frame can also consist of two frame elements, each of theseelements forming a frame itself. Typically, one such frame element willbe arranged on the cathode side, and another one on the anode sideduring assembly of the battery, and the two frame elements will bebonded together during a final assembly step to form one frame. In caseof the use of two frame elements, the separator can arranged with itscircumferential edges lying between the two frame elements. It will thusbe held in place by the two frame elements when they are bondedtogether, and any contact between the anode material and the cathode canbe avoided in a very reliable and simple way.

The first and second polymeric jacket layer may also be coated with ahot-melt adhesive on the side lying on the first and second currentcollectors, respectively. The two polymeric jacket layer can thus besealed together to form the outer jacket just by applying heat in alaminating operation.

The polymeric jacket layers can be simple polymeric sheets having asurface which is larger than the surface of the current collector onwhich it is arranged. The complete current collector will then coveredby the polymeric sheets, and the jacket formed by the two polymericsheets will thus not leave any parts of the battery accessible, exceptfor two contact tabs.

Alternatively, the polymeric jacket layers can also be formed by apolymeric frame which will typically be cut out from a polymeric sheet,and which covers the outer contour of the current collector on which itis arranged. The use of such a frame has the advantage that the wholebattery will be thinner than when complete sheets are used, and theadditional sealing is limited to those areas where such a sealing isreally critical, namely to the outer contour of the current collector.On the other hand, it is easier to obtain certain desired surfaceproperties for the battery when using a whole sheet than when usingmerely a frame. A complete sheet might thus be the preferred solutionwhen a certain adhesion or a certain appearance is needed.

The current collectors are preferably metal foils, in particular copperfoils. Contact tabs for contacting the battery from the outside can beformed integrally with such a copper foil, and there is thus no need foradditional contacts. However, the use of a metallized polymeric film orsheet instead of a metal foil is also possible. According to a preferredembodiment of the invention, at least one of the current collectors is apreformed metal foil having a depression formed in a central areathereof. Such a depression can form a receptacle for a cathode mixturewhen same is applied to the current collector, and will thus facilitatethe assembly of the battery.

Preferably, the anode material is lithium. However, it is also possibleto use another material such as zinc (Zn), cadmium (Cd), lead (Pb),hydrogen absorbing alloys or any other suitable material for the anodewithout departing from the scope of the present invention.

According to a preferred embodiment of the invention, the cathodecomprises manganese-dioxide (MnO₂) as the active material. Typically,electrolytic manganese dioxide, or EMD, will be used for this purpose.However, it is also possible to use another material such as nickelhydroxide, silver oxide, carbon monofluoride or any other suitablematerial for the cathode without departing from the scope of the presentinvention.

It should further be noted that the first and the second polymericjacket layers can be made of one single sheet which is folded in themiddle to form the outer jacket and thereby seal the battery. Theadvantage of such a solution is that the folding line already providesone side which will be perfectly sealed.

The present invention also concerns a method to make a battery accordingto claim 12. Preferred embodiments of this method arise from thedependent claims and from the description of two preferred embodimentsgiven below.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained more in detail inthe following description with reference to the drawings, wherein

FIG. 1 is a sectional view of a flat battery according to a firstembodiment of the invention;

FIG. 2 is a sectional view of a flat battery according to a secondembodiment of the invention;

FIG. 3( a) is a top view of a first preassembled part of the flatbattery of FIG. 1;

FIG. 3( b) is a top view of a second preassembled part of the flatbattery of FIG. 1

DETAILED DESCRIPTION

It will be appreciated that the following description is intended torefer to two specific embodiments of the invention which have beenselected for illustration in the drawings but which are not intended todefine or limit the invention, other than in the appended claims.

FIG. 1 shows a cross-section through a flat battery according to theinvention, whereas FIGS. 3( a) and 3(b) each show a top view of apreassembled part of the flat battery shown in FIG. 1. More precisely,FIG. 3( a) shows the upper layers of the battery shown in FIG. 1, whileFIG. 3( b) shows the lower layers of the same.

Referring to FIGS. 3( a) and 3(b), a method according to the inventionto produce the battery as shown in FIG. 1 will now be described.

Firstly, the preassembly of the lower layers as shown in FIG. 3( b) willbe described.

In a first step, a first rectangular polymeric sheet 24 being coatedwith a heat-sealable material on one side is provided. It should benoted that, although rectangular components have been chosen for theembodiments described herein, the individual components and theassembled battery can have any desired shape, e.g. rectangular withrounded edges, oval, circular etc. A first current collector 18 isarranged on the side being coated with a heat-sealable material of saidpolymeric sheet 24. Said current collector 18 is made of a rectangularcopper foil or another appropriate metal foil and has a perimeter whichis smaller than the perimeter of the polymeric sheet 24, for example bysome millimeters on all sides. On one side, it is provided with acontact tab 17 extending beyond the outer contour of the polymeric sheet24. The current collector 18 will be arranged in the centre of the firstpolymeric sheet 24 such that a frame-like outer zone of the polymericsheet 24 remains uncovered, as it can be seen in FIG. 3( a).

In the next step, a first frame element 22 a will be arranged on thefirst polymeric sheet 24 and the first current collector 18. This frameelement is a rectangular polyester frame 22 having a thickness lying Inthe order of about 100 μm and being coated with hot-melt adhesive EVA(ethylene vinyl acetate) on its upper and its lower surface. The outerperimeter of this frame element 22 a is smaller than the perimeter ofthe polymeric sheet 24, but larger than the perimeter of the currentcollector 18, whereas the inner perimeter of the frame element 22 a issmaller than the perimeter of the current collector 18. The frameelement 22 a will be arranged on the polymeric sheet 24 and the currentcollector 18 symmetrically with respect to the center of all components,such that an inner region of the frame element 22 a lies on the currentcollector 18, while an outer region of the frame element 22 a liesdirectly on the polymeric sheet 24.

These three elements, i.e. the first polymeric sheet 24, the firstcurrent collector 18, and the first frame element 22 a will be bonded toeach other. To do so, it is sufficient to apply heat and pressure, e.g.by applying a heated block, and the hot-melt coating on the polymericsheet and on the frame element 22 a will melt and adhere to the metalfoil lying in the middle. Alternatively, one can also bond these threeelements together only temporarily by applying heated pins to someselected points.

In the next step, a lithium foil which will form an anode 12 is arrangedon the current collector copper foil 18. This is normally done under alow moisture environment to protect the lithium. The anode lithium foil12 is also rectangular with its surface being slightly smaller than thesurface of the current collector 18. It will preferably be arrangedsymmetrically in the middle of the current collector 18, thereby leavinga frame-like outer region of the current collector 18 uncovered.

Now referring to FIG. 3( a), in the same manner, a second polymericsheet 26 identical to the first polymeric sheet 24 and thus also coatedwith a heat-sealable material will be provided. A second currentcollector 20 with a second contact tab 19 will be arranged on saidsecond polymeric sheet 26 in the same way as described above for thefirst polymeric sheet 24 being arranged on the first current collector18. In a next step, a second frame element 22 b identical to the firstframe element 22 a described above will be arranged on the on the secondpolymeric sheet 26 and the second current collector 20 just as describedabove. The second polymeric sheet 26, the second current collector 20,and the second frame element 22 b will then also be bonded to each otherjust as described above referring to FIG. 3( a).

Once the frame element 22 b lies on the copper foil forming the currentcollector 20 and has been bonded thereto and to the polymeric sheet 26as described above, instead of the lithium foil forming the anode 12, amixture which will form the cathode 16 is applied to the central zone ofthe current collector 20. This central zone is bordered by the frameelement 22 b which forms a wall to hold the mixture in place. Saidmixture contains preferably manganese dioxide as the active cathodematerial, but other suitable cathode materials can also be chosenwithout departing from the scope of the present invention. In additionto the active cathode material, the mixture further comprises anelectrolyte, typically a lithium salt, such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), or lithium triflate(LiCF₃SO₃), in a mixture of aprotic organic solvents, such as PC:EC(propylene carbonate:ethylene carbonate), EC:DME (ethylenecarbonate:dimethoxyethane), or EC:DMC (ethylene carbonate:dimethylcarbonate). Any other suitable electrolyte can also be used.Furthermore, the cathode mixture comprises a conductive phase to promoteelectrical conductance and to enhance utilization of the activematerial, such as conductive carbon, graphite, or another suitablematerial. The mixture further comprises a substance acting as binder tohold the different components together, such as PTFE(Polytetrafluoroethylene) or PVDF.

After the application of the cathode mixture forming a composite cathode16, a porous film serving as a separator 14, e.g. a PE or PP film, as itis well known in the art, is arranged on this cathode 16. For theexample shown here, the outer contour of the separator 14 corresponds tothe inner contour of the two frame elements 22 a, 22 b. The separatorcan also have a bigger surface than the cathode and extendcircumferentially beyond the cathode. Such a solution will in general bepreferred, because it allows to avoid that the cathode and the anodecontact each other. To be kept in place, the separator might be arrangedbetween the two frame elements described above on its outercircumference.

The preassembled unit shown in FIG. 3( b) comprising the first polymericsheet 24, the first current collector 18, frame element 22 a and theanode 12 can now be assembled to the preassembled unit shown in FIG. 3(a). To do so, one of the two preassembled units will be flipped over,and the two halves will be arranged on each other such that the twoframe elements 22 a, 22 b are aligned with each other and form one frame22. The anode 12 is thereby brought into contact with the separator 14,the outer contours of the two polymeric sheets 24, 26 now lying on topof each other. It should be noted that it is also possible to use aframe made in one piece instead of a frame comprising two frame elementsas described herein. This frame will then preferably arranged on thecathode side before the two units shown in FIGS. 3( a) and 3(b) areassembled to each other, just as it is described below for the secondembodiments shown in FIG. 2.

As shown in FIG. 1, the anode 12, separator 14 and cathode 16 then forma substantially block-shaped package 10 in the heart of the battery withthe frame 22 formed by the frame elements 22 a, 22 b circumscribing saidpackage 10. The inner contour of the frame 22 corresponds to the outercontour of the package 10, and the height h of the frame 22 correspondsto the height h of the package 10 formed by the anode 12, the cathode 16and the separator 14. The circumferentially outer area of the currentcollector 18 extending beyond the anode 12 lies on the sealing frame 22in an area being adjacent to the package 10, i.e. in ancircumferentially inner area of the frame 22, thereby partly coveringsaid frame. As one can see in FIGS. 1 and 2, approximately half of thesurface of the frame 22 is covered by the current collectors 18, 20,whereas a circumferentially outer area of the frame 22 is not covered bythe current collector 18, 20 and is in direct contact with the polymericsheets 24, 26 when the battery is assembled.

In a laminating operation the heat-melt adhesive coatings on the contactsurface of the frame elements 22 a, 22 b are melted, such that the twoframe elements 22 a, 22 b are now definitively bonded to form one frame22. If this has not happened beforehand, the frame elements 22 a, 22 bwill also be bonded to the current collectors 18, 20. At the same time,the two polymeric sheets 24, 26 being coated with a heat-sealablematerial on their inner sides contacting each other are sealed togetheron their outer circumference in an outer sealing area having a width W.The polymeric sheet 24, 26 serving as a polymeric jacket layer to formthe outer jacket will typically have a thickness of 50-75 μm. Thislaminating operation can be limited to three sides of the battery in afirst step, a fourth side being left unsealed temporarily. In this case,a vacuum will be applied in a next step to remove any air, vapor,moisture etc. from the battery. While this vacuum is applied, the fourthside will be laminated to seal the battery completely. It is alsopossible to seal all for sides in one step while evacuating the batteryat the same time, but this is slightly more difficult to handle. As thisevacuation step may take longer than the assembly steps previouslydescribed, the batteries may be grouped during evacuation and duringsealing of the fourth side, even if the previous assembly steps havebeen carried out individually, e.g. on a rotating assembly diode or on aconveyer belt.

It should be noted that it is also possible to use only one polymericsheet instead of two separate sheets 24, 26. In this case, the firstcurrent collector 18, the first frame element 22 a, and the anode 12will be applied on one half of this sheet as described above, while thesecond current collector 20, the cathode 16, the second frame element 22b and the separator 14 will be arranged on the second half just aspreviously described for two separate sheets 24, 26. The polymeric sheetwill then be folded in the middle to assemble the battery, and oneproceeds with the laminating operation as described above. One advantageof this method is that one side, namely the one which is folded, doesnot need to be sealed, and that it can be easier to align the two halvesforming the battery with each other.

FIG. 2 shows a second embodiment of a battery according to theinvention. The same parts are denoted using the same reference numerals,and in the following only the differences with respect to the firstembodiment will be described.

As one can see in FIG. 2, instead of the polymeric sheets 24, 26 shownin FIG. 1, polymeric frames 24′, 26′ are used. These have the samefunction as the polymeric sheets 24, 26 in the first embodiment. Saidpolymeric frames 24′, 26′ can be cut out in a polymeric sheet as it isused for the polymeric sheets 24, 26 of the first embodiment, and willthus also typically have a thickness of 50-75 μm. The outer contour ofthe polymeric frames 24′, 26′ shown in FIG. 2 corresponds to the outercontour of the sheets 24, 26 used for the first embodiment and shown inFIG. 1. The inner contour of the frames 24′, 26′ is slightly smallerthan the outer contour of current collectors 18′, 20′, such that theouter contour of the current collectors 18′, 20′ is covered by theframes 24, 26′ after assembly of the battery. The outer jacket formed bythe two polymeric frames 24′, 26′ is thus not completely closed, and thecurrent collectors 18′, 20′ are not covered in their center. Theuncovered centre can also serve as a contact, and contact tabs as shownin FIGS. 3( a), 3(b) are thus not absolutely necessary, when a frameinstead of complete sheets is used to form the jacket.

The current collectors 18′, 20′ are copper foils just as for the firstembodiments, but the second current collector 20′ has been preformed andis provided with a depression in its center. This depression forms areceptacle for the cathode mixture 16′, as it can be seen in FIG. 2. Thefirst current collector 18′ can also be provided with a depression toreceive the lithium foil forming the anode 12′, but as the thickness ofthe anode is rather small compared to the thickness of the cathode, theadvantages of such a depression on the lithium side are less obviousthan for the depression on the cathode side forming a receptacle for thecathode mixture.

The frame 22′ is not formed by two frame elements 22 a, 22 b asdescribed for the first embodiment above, but is merely formed by onesingle frame. This frame will be arranged on the cathode side, i.e. onthe second current collector 20′ and polymeric frame 26′ just asdescribed above for the first embodiment, and these three parts will bebonded together before application of the cathode mixture 16′. The totalthickness of the frame 22′ is smaller than the thickness of the package10′ formed by the active materials and can lie somewhere between thethickness of the separator layer 14 and the thickness of the package 10.In the final laminating step, the frame 22′, which is coated with ahot-meld adhesive on both sides, will be bonded to the first currentcollector 18′ and to the polymeric frames 24′, 26′ forming the outerjacket.

With the embodiment as shown in FIG. 2, one can obtain a battery havinga total thickness lying under the thickness one can achieve with theembodiment of FIG. 1.

One of the main advantages of the battery described herein is themultiple sealing zones. As it can be seen in FIGS. 1 and 2, acircumferentially inner part of the frame 22, 22′ is sandwiched betweenthe two current collectors 18,18′, 20, 20′ and is bonded to them via themelted hot-melt-adhesive coating of the frame 22, 22′. The currentcollectors 18, 18′, 20, 20′ and the frame 22 lying between them thusform a hermetically sealed “housing” protecting the package 10, 10′ andthus the battery's anode and cathode from the entry of moisture.

In addition to this inner sealing provided by the frame 22, 22′ sealedto the current collectors 18, 18′, 20, 20′, an outer sealing jacket isformed by the two polymeric jacket layers, i.e. by polymeric sheets 24,26 in the first embodiment, or by polymeric frames 24′, 26′ in thesecond embodiment, which are sealed to each other on their outercircumference. This jacket forms an additional protection for thecomplete battery, including the current collectors 18, 18′, 20, 20′.Furthermore, the polymeric jacket layers 24, 24′, 26, 26′ are alsosealed to the frame 22, 22′ in a region which lies circumferentiallyoutwards of the current collectors 18, 18′, 20, 20′. Finally, thepolymeric jacket layers 24, 24′, 26, 26′ are sealed to the currentcollectors 18, 18′, 20, 20′ on the complete surface of these currentcollectors. Due to the low thickness of the single layers, i.e. of thepolymeric jacket layers, the current collectors, the active materials aswell as the frame, the whole battery will remain flexible while beingperfectly sealed.

The combination of the frame sandwiched between the current collectorsin a circumferentially inner region and sandwiched between the twopolymeric jacket layers in an outer region with the jacket formed bythese jacket layers thus provides an excellent sealing for the batteryaccording to the invention.

1. A method for making a flat battery comprising the following steps:(a) providing a first polymeric jacket layer, (b) providing a firstcurrent collector and arranging same on said first polymeric sheet, (c)applying an anode material on said first current collector, (d)providing a second polymeric sheet, (e) providing a second currentcollector made of a metal foil and arranging same on said secondpolymeric sheet, (f) providing a sealing frame having an inner contourcorresponding substantially to an outer contour of the anode materialand the separator on the first current collector, (g) arranging saidframe on one of the current collectors, the frame covering the outercircumference of the current collector, (h) providing a cathode materialand applying same on the second current collector, (i) providing aseparator layer and arranging same on the cathode material, (j)assembling the flat battery by returning one of the two polymeric jacketlayers and arranging it and on the other polymeric jacket layer suchthat the separator layer is sandwiched between the cathode material andthe anode material, the sealing frame then extending circumferentiallyaround the anode material, the cathode material and the separator, and(k) sealing together said first and second polymeric jacket layer in azone extending circumferentially beyond the current collectors to forman outer jacket for the battery package, wherein said polymeric jacketlayers are also sealed to the frame in a region which liescircumferentially outwards of the current collectors.
 2. The methodaccording to claim 1, wherein the sealing frame is arranged on thesecond current collector before step h is executed.
 3. The methodaccording to claim 2, wherein the sealing frame consists of two sealingframe elements, and wherein a first frame element is applied on thefirst current collector before execution of step (c), and a second frameelement is applied on the second current collector before execution ofstep (h), and the two frame elements are bonded together to form a frameupon execution of step (k).
 4. The method according to claim 3, whereineach sealing frame element, the current collector on which it isarranged and the polymeric jacket layer on which said current collectoris arranged) are bonded to each other before the execution of steps (c)and (h), respectively.
 5. The method according to 4, wherein the sealingframe is coated with a heat-sealable material and is bonded to thecurrent collector on which it is arranged before step (j) is executed.